1. Field of the Invention
The present invention relates to a vacuum interface valve, and more particularly to a vacuum interface valve for use in a vacuum sewerage system in which sewage drained from houses is transported under the influence of a vacuum to a wastewater treatment station or a public main sewer.
2. Description of the Prior Art
Recently, a vacuum sewerage system is becoming recognized as an economical alternative to a conventional gravity sewerage system including a network of underground pipes which is constructed at a relatively high cost. The vacuum sewerage system comprises sumps with a vacuum interface valve, vacuum sewers and a vacuum collection station.
In the vacuum sewerage system, downhill flow of sewage drained from houses is collected in the sump with the vacuum interface valve. When a control device detects a predetermined liquid level of the sump, the vacuum interface valve is opened to feed sewage in the sump into the vacuum sewer. After finishing the feed of sewage in the sump, the vacuum interface valve continues to be opened for a certain period of time, during which atmospheric air is sucked into the vacuum sewer. The sewage is mixed with the expanded air in the vacuum sewer to form a mixed flow and is conveyed to the vacuum collection station. When the sewage collected in a collection tank of the vacuum collection station reaches a certain liquid level, the sewage is fed to the wastewater treatment station or the public main sewer by discharge pumps.
Next, a conventional vacuum interface valve in the vacuum sewerage system will be described with reference to FIGS. 6 and 7. The conventional vacuum interface valve V comprises a casing 21 having a substantially Y-shaped body, a valve member 22 which is moved up and down in an oblique direction with respect to an axis of the vacuum sewer, and a valve rod 23 for supporting the valve member 22. The casing 21 has an inlet 21i and an outlet 21o which extend in a horizontal direction. On the upper portion of the casing 21 there is provided a housing 25 in which a piston 26 is provided so as to perform a reciprocating motion. The valve rod 23 is formed, at the upper portion thereof, with a screw 23a. The piston 26 is fixed to the valve rod 23 by being held between a plate 27 and a nut 28 engaging the screw 23a.
A diaphragm 29 is provided between the piston 26 and the housing 25 to define two chambers, that is, a vacuum chamber 25a and an atmospheric chamber 25b. A compressive coil spring 30 is provided between the upper wall of the housing 25 and the piston 26 to urge the piston 26 obliquely downwardly.
According to the vacuum interface valve V thus constructed, in closing the valve, when the valve rod 23 is extended downwardly by the urging force of the spring 30, the valve member 22 engages a valve seat 21s formed in the casing 21 and prevents sewage from flowing from the inlet 21i to the outlet 21o.
In contrast, in opening the valve, when the vacuum chamber 25a is placed in communication with a vacuum source and the valve rod 23 is retracted by a pressure difference between the vacuum chamber 25a and the atmospheric chamber 25b, the valve member 22 is moved away from the valve seat 21s and sewage can flow from the inlet 21i to the outlet 21o.
However, in the conventional vacuum interface valve V, since a gap g between the valve member 22 and the inner wall of the casing 21 is small, foreign matters F such as gravel are caught between the inner wall of the casing 21 and the valve member 22 as shown in FIG. 7 and the valve member 22 is stuck at a partially opened position. Since the gap g is constant along a stroke of the valve member 22 to guide the valve member 22, there is a high possibility that the foreign matters F are caught everywhere during the actuation of the valve member 22.
Further, when the valve member 22 is stuck at the partially opened position by the existence of the foreign matters F, vacuum in the total sewerage system is lost, resulting in a malfunction of the system.